When, owing to an intake air quantity sensor failure, the detected intake air quantity includes an error, inappropriate fuel injection is performed. This leads to toxic substance release to the atmosphere.
For instance, Patent document 1 discloses a fault diagnosis apparatus configured to determine that an airflow sensor has a fault when an absolute value of the difference between the estimated intake air flow rate estimated by an intake air quantity estimation means and the actual intake air flow rate measured by the airflow sensor is greater than a value that the absolute value cannot usually take or a predetermined value G0 determined based on an engine rotation speed and engine load.
However, when the throttle opening becomes a low opening position or when the rotation speed of the internal combustion engine becomes low (i.e., in the operation range of low intake air quantities), even with the airflow meter unfailed, there is a risk that the actual intake air quantity greatly diverges in a direction for increasing or decreasing of a divergence value from the estimated intake air quantity owing to intake air pulsating flow.
That is, there is a problem that the fault diagnosis for the airflow meter cannot be necessarily performed accurately at particular rotation speeds owing to dispersions of the detected values of the airflow meter, which dispersions change depending upon the rotation speed of the internal combustion engine. From the viewpoint of the above, there is room for improvement in point of the enhanced fault diagnosis accuracy.